In this known method, a tyre carcass is formed by:                winding a body ply about the drum, so that two annular lateral portions of the body ply rest on the respective turn-up bladders;        fitting a respective bead bundle on each half-drum and respective annular lateral portion;        clamping the bead bundles in position on the drum; the beads defining, on the body ply, the two annular lateral portions outwards of the bead bundles, and a central portion between the bead bundles;        moving the two half-drums towards each other to form the central portion into a toroidal shape; and        once the central portion is shaped, applying air inflation pressure to expand the annular lateral portions outwards and about the respective bead bundles.        
In the above known tyre carcass forming method, simply inflating the turn-up bladders is rarely sufficient, especially in the case of extensive turn-ups, to ensure firm adhesion of the turned-up annular lateral portions to the respective sidewalls of the toroidal central portion.
Consequently, each turn-up bladder is known to be associated with at least one respective push bladder located axially outwards of the relative turn-up bladder, and which, when inflated, presses the relative turn-up bladder against the relative sidewall of the central portion of the body ply. When so pressed, the turn-up bladder expands radially outwards to increase the portion of its outer surface adhering to the relative sidewall of the central portion, and so complete turn-up of the relative annular lateral portion of the body ply.
Using push bladders poses several drawbacks, on account of each turn-up bladder being expanded radially by the relative push bladder subjecting it to severe axial thrust, which, for an average-size tyre carcass, is in the region of a few tons, and is transmitted to the toroidal central portion of the body ply.
Bearing in mind that:                the carcass is green;        the axial thrust exerted by the push bladders can only be counteracted by feeding compressed air into the toroidal central portion of the body ply; and        the greater the air pressure inside the central portion, the greater the problems posed in terms of airtightness and structural resistance of the central portion;the only way of counteracting the axial thrust of the push bladders, in the case of relatively high turn-ups, is to inflate a central bladder inside the toroidal central portion, which involves added cost and equally serious structural problems.        
In addition to structural problems, push bladders also pose size problems, by being located at least partly outwards of the relative turn-up bladders on the half-drums. As a result, the half-drums must be built long enough to support the relative push bladders, which means an increase in size, stronger drum supporting structures as a whole, and considerable added cost.
The above considerations apply even more so when, as in U.S. Pat. Nos. 5,660,677; 2,986,196, the push bladders are replaced by external hydraulically or pneumatically operated push devices. In which case, the external push devices not only greatly increase the axial size of the drum, but also make it substantially impossible to employ the drum on multidrum turrets commonly used in the tyre industry. Moreover, unlike push bladders, which subject the turn-up bladders to substantially evenly distributed stress, external push devices—normally comprising push cans—subject the turn-up bladders to more or less concentrated stress that greatly reduces the working life of the turn-up bladders.